Metalorganic chemical vapor deposition (MOCVD) is a process used to synthesize materials by means of a chemical reaction in a vapor of a metal-organic precursor. The chemical reaction breaks apart the precursor molecules and deposits metals or metals oxides on a substrate material. The organic portion of the precursor returns to the gaseous phase. The deposited metals or metal oxides may be particles with nanometer sized dimensions (and therefore, large surface area to mass ratios)
It is often desirable that the MOCVD reactions occur on a specific substrate surface, such that the surface is covered by discrete particles, a network of particles, or a continuous film of deposited material. Previously reported MOCVD methods use a flowing stream of the precursor vapor, which may be mixed with an inert carrier gas. The reactant stream flows over or through the substrate. The substrate is often heated, so as to induce the chemical conversion of molecules of precursor which collide with the substrate surface from the vapor phase. This heating may be applied to the entire deposition chamber (“hot-wall” CVD) or may be applied only to the substrate by means of inductive heating (“cold wall” CVD). A catalytic “hot wire” or a plasma may also be present to encourage the deposition reaction.
Convection based MOCVD methods show poor performance in depositing uniform coatings on powders and highly porous materials/structures, which may have a very large surface areas and do not present all available surfaces to a convective reactant flow. Some methods devised to cope with this problem for powdered materials are fluidized bed reactors that agitate the substrate powder by flowing the reactant vapor through the powder via a fritted disc. However, to date known methods devised to deposit metal or metal oxide particles on powders and highly porous materials/structures are inefficient with respect to the use of the metal-organic precursors and/or complex and expensive in design, inhibiting the commercial viability of such methods. A method for efficiently depositing metal/metal oxide particles on powders and highly porous materials/structures was therefore developed.
Metal or metal oxide nanoparticles supported on the surfaces of high surface area materials/structures are required for the catalysis of chemical reactions in many chemical and electrochemical devices. For both chemical and electrochemical devices, these functional metal or metal oxide particles are dispersed on supporting materials. In the case of chemical devices, the supporting material is usually a chemically and thermally stable material with high surface area. For electrochemical devices, the functional metal or metal oxide particles are typically dispersed on electrically conductive supports or inside highly porous electrodes which conduct both electrons and ions, and have high gas permeability.
Chemical devices containing functional metal or metal oxide materials perform a multitude of heterogeneous reactions. Such reactions include fuel reforming (e.g. steam reforming, partial-oxidation, auto-thermal), hydrolysis (e.g., hydrogen production from water), hydrogenation (e.g., reaction of hydrogen with fats to form margarine), and more generically, catalytic reduction (e.g. ammonia formation from nitrogen) and oxidation (e.g., water gas shift and nitric acid formation from ammonia) reactions. Highly dispersed metal or metal oxide catalyst particles with high surface areas are desirable as increased catalyst surface area typically increases overall device reaction rates.
Electrochemical devices with functional metal or metal oxide materials in their electrodes are used in variety of applications including batteries, fuel cells, hydrogen pumps, electrolysis cells, supercapacitors, sensors, hydrogen separation membranes and membrane reactors. Highly dispersed and high surface area metal or metal oxide particles in the electrodes of electrochemical devices are often essential in promoting fast reaction and intercalation rates, and more generally, the fast charge transfer rates measured in highly active electrodes. The overall functionality of most electrochemical devices is then highly dependent on the chemical and physical nature of the functional metal or metal oxide particles.
In particular, the chemical and physical nature of the functional metal or metal oxide particles in fuel cell electrodes are critical to high performance and fuel conversion efficiencies, and therefore critical to the overall commercial viability of a fuel cell system. Fuel cells are attractive alternatives to combustion engines for power generation, because of their higher efficiency and the lower level of pollutants produced from their operation. A fuel cell generates electricity from the electrochemical reaction of a fuel, e.g. methane, methanol, gasoline, or hydrogen, with oxygen normally obtained from air.
There are three common types of fuel cells i.e., 1) direct hydrogen/air fuel cells, in which hydrogen is stored and then delivered to the fuel cell as needed; 2) indirect hydrogen/air fuel cells, in which hydrogen is generated on site from a hydrocarbon fuel, cleaned of carbon monoxide, and subsequently fed to the fuel cell; and 3) direct alcohol fuel cells, such as methanol (“DMFC”), ethanol, isopropanol and the like, in which an alcohol/water solution is directly supplied to the fuel cell. An example of this later fuel cell was described, for example, in U.S. Pat. No. 5,559,638, the disclosure of which is incorporated herein by reference.
Regardless of the fuel cell design chosen, the operating efficiency of the device is partly limited by the efficiency of the electrolyte at transporting ions (e.g., protons, oxygen vacancies, hydroxyl ions, bicarbonate ions, etc.). Typically, perfluorinated sulfonic acid polymers, sulfonated hydrocarbon polymers, and composites thereof are used as electrolyte membrane materials for proton conducting fuel cells. However, these conventional materials utilize hydronium ions (H3O+) to facilitate proton conduction. Accordingly, these materials must be hydrated, and a loss of water immediately results in degradation of the conductivity of the electrolyte and therefore the efficiency of the fuel cell.
As a result, fuel cells utilizing these materials require peripheral systems to regulate water evaporation rates. If water is flushed from the system too quickly, the system will dry out, and the conductivity of the polymer electrolyte will decrease. If water is removed too slowly, liquid water can form and flood the porous volumes within the electrodes, blocking the access of gaseous species. These peripheral systems increase the complexity and cost of these fuel cells, from the use of expensive noble catalysts (Pt) to temperature requirements that cannot exceed much above 100° C. As a result of these temperature limitations, the fuel cell catalysts and other systems cannot be operated to maximum efficiency. Higher temperatures can also reduce carbon monoxide poisoning of the fuel cell catalyst.
It has been shown that the solid acids such as CsHSO4 can be used as the electrolyte in fuel cells operated at temperatures of 140-160° C. (Haile, S. M., et al. Nature 2001, 410, 910-913). The high conductivity of CsHSO4 and analogous materials results from a structural phase transition (referred to as a superprotonic phase transition) that occurs at 141° C. from an ordered structure, based on chains of SO4 groups linked by well-defined hydrogen bonds, to a disordered structure in which SO4 groups freely reorient and easily pass protons between one another. Across this transition, the proton conductivity increases by 3 to 4 orders of magnitude from 10−6 Ω−1cm−1 (phase II) to 10−3-10−2 Ω−1cm−1 (phase I; Baranov, A. I., et al. JETP Lett. 1982, 36(11), 459-462). Thus, disorder in the crystal structure is a key prerequisite for high proton conductivity.
However, the lifetime of these sulfate and selenium based “superprotonic” solid acids is short (Merle, R. B., et al. Energy & Fuels 2003, 17, 210-215) when operated under standard fuel cell conditions. The short lifetime of both CsHSO4 and CsHSeO4 under fuel cell operating conditions results from the reduction of sulfur and selenium by hydrogen in the presence of typical fuel cell catalysts, according to:2CsHSO4+4H2→Cs2SO4+H2S+4H2O2CsHSeO4+4H2→Cs2SeO4+H2Se+4H2O
Accordingly, research was done to find a “superprotonic” solid acid electrolyte stable under fuel cell conditions, resulting in the demonstration that CsH2PO4 (CDP) has as superprotonic transition and is stable under fuel cell conditions (Boysen, D. A., et al. Science 2004, 303, 68-70). For CsH2PO4, the superprotonic transition is at 231° C., above which the material has a high proton conductivity: e.g., 2.5×10−2 Ω−1cm−1 at 250° C. Solid acid fuel cells (SAFCs) using CsH2PO4 then operate at intermediate temperatures (˜230-280° C.), are inherently impermeable to gases, and transport “bare” protons through a solid electrolyte. These properties give SAFCs advantages over other fuel cell technologies in cost, durability, start/stop cycling, fuel flexibility, and simplified system design. To date, solid acid fuel cells (SAFCs) utilizing this electrolyte as thin (10-25 μm) gas tight electrolyte layers have demonstrated peak power densities of over 330 mW/cm2 on hydrogen/air with lifetimes of thousands of hours. Moreover, SAFC stacks have demonstrated robustness to thermal cycling, power outputs of over 1 kW, and degradation rates similar to those of single cells suggesting stack lifetimes in the thousands of hours.
SAFCs have also demonstrated very high tolerances to typical anode catalyst poisons such as carbon monoxide (CO), ammonia (NH3), and hydrogen sulfide (H2S): measured tolerances are 20%, 100 ppm, and 100 ppm, respectively, without significant performance decreases. These high impurity tolerances and the intermediate operating temperatures allow SAFC power systems to operate on reformed fuels with simplified systems. Taken together, the advantageous properties of SAFCs are anticipated to result in relative low SAFC stack and system costs, specifically because of: 1) easy cell and stack fabrication, 2) durable on/off cycling, 3) standard metal and polymer stack components, and 4) simplified systems. SAFCs thus provide the vast majority of the benefits of both high and low temperature systems, but few of their disadvantages. That is, SAFCs operate at high enough temperatures to run effectively on a wide range of reformed fuels, but not so hot that the thermal stability/cost of stack and system components limits commercial viability.
To achieve the high performance and stability of current SAFCs, it was necessary to increase the interaction between the catalyst particles (typically, platinum) used in SAFC electrodes and the solid electrolyte while maintaining reactant gas access, as catalytic reactions only take place on catalyst particles both in contact with the electrolyte and reactant gases. This motivated the development of a deposition method to place the catalyst particles directly on the surface of the solid acid electrolyte. As the electrolyte is water soluble, most common aqueous methods for creating and depositing catalyst particles on substrate materials would not be suitable for use with solid acid electrolytes. Therefore, the simple method described herein for depositing catalyst particles from the gas phase was developed.
This method is applicable to a broad range of chemical and electrochemical devices due to the simple and effective manner in which metal and metal oxide particles can be deposited onto the surfaces of electrodes, high surface area powders, and other porous materials typically used in such devices.